And indeed it certainly looks the part. We've sent on this image to the Museum's meteorite expert Dr Alex Bevan, Head, Earth & Planetary Sciences. He explains:

It is a natural concretion of so-called ‘iron-stone’. Basically, iron-stone concretions (sometimes called iron-stone nodules) can form over geological time scales in a variety of rock types. Because they are often harder than the rocks in which they formed, they can survive long after the host rock has been eroded away. In this way they come to lie on apparently unrelated surfaces and so appear ‘exotic’.

Because of their unusual appearance they are often mistaken for meteorites. They are made predominantly of iron oxides (in this case possibly hematite or goethite) that were deposited from iron-rich percolating waters. The surface is slightly botryoidal, meaning it is slightly globular in appearance. This is typical of the iron oxide minerals hematite and goethite.

So thank you @Surfn_Stuff for sending in the image, and we hope you enjoy the explanation.